1. Field of the Invention
This invention relates to traffic management in communications systems. More specifically, this invention relates to management of access channel traffic in a communications system.
2. Background and Related Art
A communications system comprises a communications network and a set of nodes that communicate with the network. The communications links between the network and the nodes may be wired and/or wireless. The network may also communicate with other networks, such that a node may communicate with an entity within the network, with another node connected to the network, and/or with an entity and/or a node on another network.
One example of a communications network is a local-area network (LAN), where the network may include a set of servers and the individual nodes may include workstations, personal computers, and/or peripheral devices such as storage units and printers. Another example of a communications network is a wireless network for cellular communications, where the network may include a set of base stations and administrative units (such as mobile service controllers (MSCs) and location registers) and the individual nodes may be mobile units that communicate with the base stations over a radiolink. A mobile unit may be a cellular telephone, a wireless modem connected to a computer or other data-generating device, or a wireless local loop (WLL) station. Through the base stations, the mobile units may communicate with each other and/or with devices on other networks such as the Internet and/or the public switched telephone network (PSTN).
As shown in FIG. 1, a number of nodes 100 may transmit information to a network 200 over a common channel 250. In a local-area network, for example, several workstations or personal computers may attempt to send information to the network across the same Ethernet connection. In a system for mobile wireless communications, such as a cellular telephone system, several users may attempt to gain access to the network at the same time by sending access requests over a common access channel (e.g. according to an ALOHA scheme). When transmissions from two or more users are received by the network over the same channel at the same time, a data collision may result, which prevents any of the transmissions from being received correctly. As a consequence, the colliding transmissions must be retransmitted separately until each one has been received correctly, thereby increasing channel traffic and causing system delays.
It is possible for a node transmitting over a wired link to receive information regarding a data collision as it occurs. For a typical node transmitting over a wireless link, however, meaningful information concerning current channel use may be obtained only indirectly from the network. Moreover, such information is typically received only in the negative, such as in the form of a timeout (i.e., a failure to receive acknowledgement of the transmission within a specified time period). Due at least in part to this feedback delay, data collisions caused by channel overuse become even more costly in wireless systems.
Systems using ALOHA schemes are particularly susceptible to data collisions. This susceptibility may be reduced somewhat by implementing a slotted ALOHA scheme instead. Under a slotted ALOHA scheme, time is divided into a series of adjacent and non-overlapping slots, and the nodes are constrained to begin transmissions only at slot boundaries.
It may be desirable to support messages of different lengths over a common channel. In a slotted ALOHA system, for example, it may be desirable to support messages that occupy several consecutive slots. Such a modification increases the complexity of the system, however, and susceptibility to collisions increases with message length.
One alternative to a single, collision-prone access channel is a reserved access channel, which a node may use only during slots it has reserved in advance. In the IS-2000 CDMA system, for example, each reverse link enhanced access channel may be designated to be either a basic access channel (i.e., slotted ALOHA) or a reserved access channel. FIG. 2 shows a block diagram of an exemplary system having a basic access channel 252a and a reserved access channel 252b on the reverse link.
While implementing a reservation mode on an access channel eliminates the possibility of collisions on that channel, negotiation of the reservation creates delays and extra channel traffic. Additionally, a node must then wait for its reserved slot(s) to arrive. Therefore, it is desirable to transmit as many messages as possible using a basic access channel. At the same time, it is desirable to manage traffic between several access channels (e.g., between channels 252a and 252b in FIG. 2) in such a way as to minimize delays.